1. Field of the Invention
The invention relates to radiation-curable compositions, and more particularly, to radiation-curable compositions having a high cure speed. The improved radiation-curable compositions have superior curing characteristics and are therefore useful as coating materials for various types of substrates including plastics, wood, porcelain, glass, paper, and the like, and as an optical molding material, three-dimensional rapid prototyping molding material, printing plate material, and the like. The improved radiation-curable compositions are well suited for use as radiation-curable coating compositions on glass-optical fibers. The improved radiation-curable compositions also demonstrate superior adhesion characteristics when applied to a variety of substrates and can be useful as general radiation-curable adhesives or more particularly as adhesives in the production technology for digital versatile disks (DVD).
2. Description of Related Art
Radiation-curable compositions have been used to provide coatings for optical glass fibers. Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating(s) is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating(s), which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
Such primary coating systems are typically prepared from radiation-curable optical glass fiber coating compositions (hereinafter referred to as xe2x80x9cradiation-curable compositionxe2x80x9d). It is a characteristic of such systems that the curing proceeds upon exposure to a radiation source, typically a UV actinic radiation source, for a time sufficient to provide a full cure of the coating compositions at the level of intensity of such source. Examples of typical radiation-curable coatings are described in published European patent application No. 566801, published PCT application WO-A-9103499, and U.S. Pat. Nos. 5,146,531, 5,219,896 and 5,336,563.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable compositions for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation-curable composition are exceeded, the radiation-curable composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable composition. The linear line production speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased the amount of radiation exposure to the radiation-curable composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable composition is undesirable and must be avoided because then the desired protective properties of the incompletely cured primary coating may not be achieved and/or the incompletely cured primary coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present. There may also be an increase in the extractables (undesirable) in the supposedly-cured coating.
In general, radiation-curable inner primary coating compositions cure at a significantly slower rate than radiation-curable outer primary coating compositions. It is believed that the reduced number of radiation-curable functional groups present in inner primary compositions compared to outer primary compositions contributes to the slower cure speed of inner primary coatings. While there exists a greater need for improving the cure speed of the inner primary coating, the need for fast curing speeds of radiation-curable compositions employed in each of the multiple coating layers surrounding optical fibers must be met.
Although line speeds have increased over the years, it is believed that a radiation-curable inner primary coating composition, having a cure speed such that 95% or more of the maximum attainable tensile modulus is attained at an irradiation dose of about 0.30 J/cm2 or less at a thickness of 75 microns, is difficult to achieve.
Such a high cure speed radiation-curable composition would also be useful in other applications where cure speed is of importance.
Radiation-curable compositions having fast cure speed can be particularly useful in stereolithography or rapid prototyping processes. Rapid prototyping involves successive curing of defined areas of resin layers on top of each other to make a three-dimensional form that can be used as a prototype, or for making a prototype. A prototype, or model, is often used in order to prepare for making new products.
An example of a rapid prototyping process is the Cubital process described in U.S. Pat. No. 5,031,120 (Pomerantz). In the Cubital process, light is passed through an erasable mask to solidify a layer of a radiation-curable, resin composition in selected areas. The non-solidified portions are removed and replaced by a removable support material, such as wax. Additional layers are added until the desired three-dimensional object is completely formed. The removable support material is commonly a wax which can be removed by melting or dissolving to provide the thereby formed, free three-dimensional object.
Another example of a rapid prototyping process is disclosed in U.S. Pat. No. 4,575,330 (Hull). The Hull process is a scanning method, in which a concentrated beam of ultraviolet light is focused on the surface of a container filled with a liquid radiation-curable, resin composition.
The light beam, moving under computer control, scribes a layer of the object onto the surface of the liquid. Wherever the beam strikes the surface, a very thin layer of the radiation-curable, resin composition is crosslinked to form a solid. To make a three-dimensional object, the entire operation is repeated, with the position of the object shifted slightly each time, whereby the object is built up layer by layer.
Examples of radiation-curable, resin compositions that have been used in rapid prototyping methods are disclosed in U.S. Pat. Nos. 5,418,112 and 5,434,196. There is always a need for a faster curing composition for use in rapid prototyping processes to decrease production time.
Radiation-curable compositions having fast cure speed can also be useful in the production of digital versatile disks (DVD). Digital versatile disks may be created by variations on a few basic processes, as disclosed, for example, by U.S. Pat. No. 4,310,919 and U.S. Pat. No. 4,423,137, the complete disclosures of which are fully incorporated herein by reference. Three technologies are currently employed for DVD bonding, namely contact adhesives, cationic or PSA UV bonding, and free radical UV bonding. The formulations must provide adhesion between the aluminum and poly carbonate layers, the gold and polycarbonate layers, and the lacquer and the polycarbonate layers. Furthermore, the adhesive coatings must have a high cure speed. However, strong, long-lasting adhesion between DVD component layers, without compromising the other desirable properties such as good release from molds or the optical properties of the polycarbonate, is not achievable with the existing systems. For example, coatings which possess strong adhesion to metallic surfaces may do so at the expense of good release or desirably fast cure speed.
Radiation-curable compositions demonstrating fast cure speed and strong adhesion to a wide variety of substrates can also be useful as general-use radiation-curable adhesives. Typically such adhesives can be employed in the manufacture of coated articles or laminated articles such as eye-glass lenses or automobile safety glass. Other uses for radiation-curable adhesives range from laboratory equipment manufacture, surgical instrument applications, and any general use for a radiation-curable adhesive used under circumstances requiring fast cure speed. Conventional adhesives, not cured by exposure to radiation, frequently demonstrate problems of setting too quickly without allowing time for adjustment of physical placement of the substrate to which the adhesive is applied. In safety-glass or other laminated glass applications, air bubbles often are formed and may become trapped between the layers of glass. Such errors in production can not be easily corrected using conventional adhesion processes and thus result in considerable delays in production and the costly waste of materials. In contrast, radiation-curable adhesives used in such applications permit any necessary adjustments of the substrates in relation to each other prior to curing the radiation-curable adhesive by exposure to radiation.
A high cure speed radiation-curable composition would also be useful for forming protective or decorative coatings on substrates, such as wood, glass, paper, porcelain and plastics. For example, a high cure speed radiation-curable composition would be very desirable for use as a coating on a wood or vinyl (plastic) floor. The faster curing coating would generally require less exposure to UV light to cure and the coated floor would be ready for use in a shorter time period.
An object of the present invention is to provide a radiation-curable composition which exhibits an enhanced cure speed.
Another object of the present invention is to provide high cure speed radiation-curable compositions which are adapted for general uses as well as specifically designed coatings including inner primary optical glass fiber coating compositions, outer primary optical glass fiber coating compositions, inks, ribbons and other matrix material layers used for coating optical glass fiber assemblies.
Another object of the present invention is to provide high cure speed radiation-curable compositions which are adapted for use as stereolithography or rapid prototyping coating compositions.
Another object of the present invention is to provide high cure speed radiation-curable compositions which can be used as general adhesives for a wide variety of substrates. In particular, such compositions can be adapted for use as specialized adhesives necessary for the production of digital versatile disks (DVD).
Another object of the present invention is to provide a radiation-curable composition which has a cure speed such that 95% or more of the maximum attainable tensile modulus is achieved using an irradiation dose of about 0.30 J/cm2 or less at a thickness of 75 microns.
It has now been found, that by formulating a radiation-curable composition to contain an unusually large amount of what is now understood to be a cure speed enhancing radiation-curable monomer or oligomer, surprisingly the cure speed of a desired radiation-curable composition can be greatly increased.
Surprisingly, the above objectives and other objectives can be achieved by formulating a radiation-curable composition to contain a cure speed enhancing radiation-curable monomer or oligomer having at least one functional group capable of polymerization in the presence of radiation being directly connected to a propoxy group, the cure speed enhancing radiation-curable monomer or oligomer being present in an amount sufficient to provide the radiation-curable composition with a cure speed such that 95% or more of the maximum attainable tensile modulus is attained at an irradiation dose of about 0.30 J/cm2 or less at a thickness of 75 microns.
Also provided is a coated substrate including a substrate and a coating on the substrate formulated from a radiation-curable coating including a cure speed enhancing radiation-curable monomer or oligomer having at least one functional group capable of polymerization in the presence of radiation being directly connected to a propoxy group. The cure speed enhancing radiation-curable monomer or oligomer should be present in an amount sufficient to provide the radiation-curable composition with a cure speed such that 95% or more of the maximum attainable tensile modulus is achieved using an irradiation dose of about 0.30 J/cm2 or less at a thickness of 75 microns.